In general, a propulsion system of a ship includes a drive shaft connected to an engine and protruding from a tail of the hull, and a propeller mounted to an end portion of the drive shaft to generate drive force by rotation. As the propeller rotates in the fluid, a pressure difference occurs between the fluid-inflow side and the fluid-outflow side, and accordingly lift force is generated at each blade. Such lift force generated at the propeller acts as drive force of the ship.
According to the above-described propulsion mechanism of the ship, underwater noise is generated from the engine and the propeller of the ship. Because underwater noise has transmission power and transmission speed several times higher than atmospheric noise, an influence exerted on the marine ecosystem is large. As such, because underwater noise generated from ships has a negative influence on the marine ecosystem in the sea route and aquaculture industry, efforts for reducing underwater noise are needed.
Recently-built ships have soundproofing equipment for reducing engine noise generated inside the ship, however, there is no particular countermeasure against propeller noise generated outside the ship. Especially, propeller noise is primarily generated by high-speed ships or submarines. Further, because international measures to control underwater noise are taken, methods for reducing propeller noise are urgently needed.